Many modern vehicles, including automobiles, light trucks, medium trucks, heavy trucks, busses, vans, delivery trucks, tractor trailer trucks, personal vehicles, commercial vehicles, etc., employ power assistance to facilitate manual steering.
While automobiles and lighter weight vehicles often employ power assisted steering systems, such power assisted steering systems have increased importance in applications involving trucks and larger commercial vehicles. Among other things, the steering demands imposed on the driver of the vehicle increases substantially as the size and weight of the vehicle increases (becoming very substantial for common trucks and large commercial vehicles), such that in many cases even with substantial power assistance, the required steering forces can be substantial.
Over the years, a variety of power assisted steering systems have been developed. These systems have utilized a number of power methodologies, such as, e.g., hydraulic power, electronic power and/or the like.
By way of example, a number of illustrative background systems are shown in the following documents, the entire disclosures of which are incorporated herein by reference:                a) U.S. Pat. No. 6,041,807, which shows a “[f]low control device of a power steering apparatus;”        b) U.S. Pat. No. 5,786,674, which shows a “[h]ydraulic servo control[,] particularly hydraulic power steering system for motor vehicles;”        c) U.S. Pat. No. 5,564,516, which shows “a clutched electric motor steering system;”        d) U.S. Pat. No. 5,192,196, which shows a “[f]low control orifice for parallel flow fluid supply to power steering gear;”        e) U.S. Pat. No. 4,862,366, which shows a “[m]otor-driven power steering system for a vehicle;”        f) Japanese Patent Publication No. 11107936 A, which apparently shows a “hydraulic unit” in which “[a]t low speed running time, the oil hydraulic pump 7 is driven by only the electric motor 14, and at high speed running time, the oil hydraulic pump 7 is driven by only the engine 13.”        g) Japanese Patent Publication No. 2002/255052 A, which apparently shows an “auxiliary machine drive device for [a] vehicle” having “two pump drive sources of the engine E and the motor M for driving a fluid pump of the auxiliary machine.”        
The demand confronted by a vehicle varies significantly depending on the size and weight of the vehicle. In this regard, there are a variety of vehicle types and classifications. By way of example, the (2001) vehicle classification system of the Federal Highway Administration (FHWA) of the United States Department of Transportation sets forth an exemplary methodology for identifying vehicle types, including, e.g.: class 1 (motorcycles); class 2 (passenger cars); class 3 (Other Two-Axle, Four-Tire Single Unit Vehicles); class 4 (buses); class 5 (Two-Axle, Six-Tire, Single Unit Trucks); etc., including classes of trucks up to class 13 (Seven or More Axle Multi-Trailer Trucks). Alternatively, trucks have been, in some instances, classified into categories of light, medium and heavy, with some illustrative weight classes as listed below in Table 1 (e.g., from the Washington State Department of Transportation).
TABLE 1Vehicle Truck ClassificationCategoryClassGVWRRepresentative VehiclesLight10-27kNpickup trucks, ambulances,0-6,000lbs.parcel delivery227-45kN(6,001-10,000lbs.)345-62kN(10,001-14,000lbs.)Medium462-71kNcity cargo van, beverage(14,001-16,000lbs.)delivery truck, wrecker,571-87kNschool bus(16,001-19,500lbs.)687-116kN(19,501-26,000lbs.)7116-147kN(26,001 to 33,000lbs.)Heavy8147 kN and overtruck tractor, concrete(33,000 lbs. and over)mixer, dump truck, firetruck, city transit busGross Vehicle Weight Rating (GVWR): weight specified by manufacturer as the maximum loaded weight (truck plus cargo) of a single vehicle.
Conventionally, for trucks and the like vehicles, a single engine driven hydraulic pump is typically provided for power steering assist that is sized to have a sufficient output at high demand times (e.g., where vehicle speeds are near zero). However, in such conventional truck and the like systems, although there is a substantially reduced demand for steering power at highway speeds, the conventional systems actually provide a higher level of steering assist fluid flow at higher engine speeds than at lower engine speeds. This reduces system efficiency because the extra fluid flow is required to be dumped across a flow control device. By way of example, as described in, e.g., U.S. Pat. No. 5,192,196 incorporated by reference above, “[c]onventionally a [flow control device has a] flow control valve includes a spool slidable in a cylinder, a port connected to the pump outlet, a bypass port, a spring urging the spool to close the bypass port, an orifice connecting the pump outlet and the steering gear, and a passage connecting steering system pressure downstream from the orifice to an end of the spool. A pressure force develops on the spool due to this feedback pressure tending to combine with the spring force to close the bypass port. These spool forces are opposed by a force on the spool resulting from pressure upstream from the orifice tending to open the bypass port. Therefore, as pump flow rate increases, the pressure differential across the orifice increases and the spool moves in the valve cylinder against the spring force to open progressively the bypass port. As the bypass port opens,” flow is reduced across the orifice and the differential pressure reduces tending to close the bypass port and maintain a controlled system flow. In conventional systems, the excess fluid flow provided at these higher engine speeds is, thus, dumped through such a control valve back to an inlet side of the pump, resulting in increased loads on the pump and a waste of engine power.
There is a substantial need for improved power assisted steering mechanisms, and, in particular, for improved mechanisms for use with trucks and commercial vehicles, which overcome the above and/or other problems in existing systems.